Sealing anodized aluminum

ABSTRACT

IN THE PROCESS FOR TREATING THE SURFACE OF ALUMINUM OR AN ALUMINUM ALLOY WHICH COMPRISES SUBJECTING SAID SURFACE TO AN ANODIC OXIDATION AND SUBSEQUENTLY SEALING WITH HOT WATER OR STEAM, THE IMPROVEMENT WHICH CONSISTS ESSENTIALLY OF SEALING SAID SURFACE BY APPLYING AN AQUEOUS SOLUTION CONTAINING FROM 0.0005 TO O.05 GM. PER LITER OF A HYDROXY CARBOXYLIC ACID SELECTED FROM THE GROUP CONSISTING OF (A) CITRIC ACID, TARTARIC ACID, GALLIC ACID AND SACCHARIC ACID, (B) A WATER SOLUBLE SALT OF SAID ACIDS OF (A), AND (C) THE MIXTURES THEREOF AT A TEMPERATURE RANGING FROM 90* C. TO THE SOLUTION BOILING POINT TEMPERATURE AND AT A PH OF FROM 5 TO 6 TO THE ANODIC OXIDIZED SURFACE.

"United States Patent 3,838,023 SEALING ANODIZED ALUMINUM Wolfgang Friedemann, Neuss, and Hans Gunther Germscheid, Hoscl, Germany, assignors to Henkel & Cie

GmbI-I, Dusseldorf-Holthausen, Germany No Drawing. Filed Dec. 5, 1972, Ser. No. 312,341

Claims priority, application Germany, Dec. 17, 1971, P 21 62 674.4 Int. Cl. C23b 9/02 U.S. Cl. 204-35 N 6 Claims ABSTRACT OF THE DISCLOSURE In the process for treating the surface of aluminum or an aluminum alloy which comprises subjecting said surface to an anodic oxidation and subsequently sealing with hot water or steam, the improvement which consists essentially of sealing said surface by applying an aqueous solution containing from 0.0005 to 0.05 gm. per liter of a hydroxy carboxylic acid selected from the group consisting of (A) citric acid, tartaric acid, gallic acid and saccharic acid, (B) a water soluble salt of said acids of (A), and (C) the mixtures thereof at a temperature ranging from 90 C. to the solution boiling point temperature and at a pH of from to 6, to the anodic oxidized surface.

PRIOR ART To protect aluminum or aluminum alloys against corrosion, anodically produced oxide layers are frequently applied to aluminum surfaces. These oxide layers protect the aluminum surfaces from the effects of the weather and other corroding media. Further, the anodic oxide layers are also applied in order to obtain a harder surface and therewith to give the aluminum an increased resistance to wear. In particular, decorative effects can be attained by the self coloring of the OXide layers or can be attained in part by their easy colorability.

A number of processes are known for the application of anodic oxide layers to aluminum. For example, the production of the oxide layers takes place using direct current in solutions of sulfuric acid (the direct current-sulfuric acid process). However, solutions of organic acids, such as in particular sulfophthalic acid or sulfanilic acid or mixtures of these organic acids with sulfuric acid, are also frequently used. The last named processes are particularly known as the autocolor processes.

These anodically applied oxide layers, however, do not fulfill all requirements with respect to protection against corrosion, since they have a porous structure. For this reason it is subsequently necessary to seal the oxide layers by after-sealing which is often effected with hot or boiling Water or steam and is known as sealing. This closes the pores and therefore considerably increases the corrosion protection.

During the subsequent consolidation of anodically applied oxide layers, however, not only are the pores closed, but a substantially thick and velvety film may also be formed over the whole surface. This velvety film is the socalled sealing coating and consists of amorphous aluminum hydroxide which is not resistant to handling, so that the decorative effect of the layer is thereby impaired. Furthermore, it reduces the adhesive strength during the bonding of such aluminum parts and, due to the increased effective surface, this sealing film promotes later soiling and corrosion. For these reasons it has previously been necessary to remove the coating by hand, mechanically or chemically.

It is already known to detach the film from sealed surfaces covered with a sealing film by a further mineral acid treatment. With this process, therefore, a further treatment step is needed, and moreover it necessitates a Patented Sept. 24, 1974 very careful treatment with the mineral acid in order to avoid damage to the oxide layer. Further, it is also known to prevent formation of sealing films by carrying out a sealing with solutions which contain nickel acetate and lignin sulfate. This method has the disadvantage that the oxide layers may become yellowed under the influence of light. Finally it is known from U.S. Pat. No. 3,672,966 and 3,657,077 to prevent the formation of sealing films without impairing the anodic oxide coating or the quality of the after-sealing by applying a solution of polyacrylates or specified dextrins to the surface. These processes have proved satisfactory. In some cases, however, especially if not carefully carried out, it is possible that residues may remain upon drying. These are undesirable, but they can be removed by a further rinsing.

OBJECTS OF THE INVENTION "It is an object of the present invention to provide a development in the process for treating the surface of aluminum or an aluminum alloy which comprises subjecting said surface to an anodic oxidation and subsequently sealing with hot water or steam, the improvement which consists essentially of sealing said surface by applying an aqueous solution containing from 0.0005 to 0.05 gm. per liter of a hydroxy carboxylic acid selected from the group consisting of (A) citric acid, tartaric acid, gallic acid and saccharic acid, (B) a water soluble salt of said acids of (A), and (C) the mixtures thereof at a temperature ranging from C. to the solution boiling point temperature and at a pH of from 5 to 6, to the anodic oxidized surface.

Other and further objects of the invention will become apparent as the description thereof proceeds.

DESCRIPTION OF THE INVENTION The present invention is directed to a development in the process for treating the surface of aluminum or an aluminum alloy which comprises subjecting said surface to an anodic oxidation and subsequently sealing with hot water or steam, the improvement which consists essentially of sealing said surface by applying an aqueous solution containing from 0.0005 to 0.05 gm. per liter of a hydroxy carboxylic acid selected from the group consisting of (A) citric acid, tartaric acid, gallic acid and saccharic acid, (B) a water soluble salt of said acids of (A), and (C) the mixtures thereof at a temperature ranging from 90 C; to the solution boiling point temperature and at a pH of from 5 to 6, to the anodic oxidized surface.

More particularly, the present invention provides a process for the treatment of a surface of aluminum or an aluminum alloy comprising subjecting said surfaces to anodic excitation and consolidating same at a temperature of from 90 C. and solution boiling temperature and at a pH value of from 5 to 6 with a solution which contains from 0.005 to 0.05 gm. per liter of a hydroxy carboxylic acid selected from the group; citric acid, tartaric acid, gallic acid and saccharic acid or a water soluble salt thereof.

Suitable sealing solutions are those aqueous solutions containing 0.0005 to 0.05 gm. per liter of an aliphatic or an aromatic hydroxy carboxylic acid, for example, citric acid, tartaric acid, gallic acid and saccharic acid.

The process may be carried out using the commercial acids or their water-soluble salts, for example, the corresponding alkali metal salts such as sodium or potassium salts, ammonium salts, and alkaline earth metal salts such as magnesium, calcium or barium salts.

The said substances are used in amounts of 0.0005 to 0.05 gm. per liter, calculated as free acids. The use of greater concentrations has been found disadvantageous, since the quality of the sealing is reduced.

A preferred form of the process comprises carrying out the sealing with a solution which contains from 0.0005 to 0.005 gm. per liter of citric acid. Instead of the citric acid, its water soluble salts may also be used in equivalent amounts.

The solutions of the acids or salts according to the invention are adjusted to a pH value of 5 to 6. This adjustment may be effected using ammonia or acetic acid. For the preparation of the solutions it is advantageous to use completely desalted, distilled or condensed water.

The sealing treatment with the solutions according to the invention is carried out at temperatures between 90 C. and the boiling point temperature. Preferably, a temperature of from 90 C. to 100 C. is utilized. The treatment time is maintained within the usual operating time schedule and preferably amounts to about 1.5 to 3.5 minutes per ,1. layer thickness of the anodic oxide layer.

Further, additives known for this purpose such as nickel or cobalt acetate may also be added in small amounts to the sealing solutions.

The advantages of the process of the present invention include preventing the formation of sealing films without the anodic oxide layer being impaired or the quality of the sealing being reduced. A rinsing after the sealing step or a spraying, in order to remove any residues from the surface, is not necessary. The appearance of the surface is not affected by the process of the invention; the appearance remains the same as that obtained by the pretreatment and the anodization. In addition the process is economical since the amounts of additives required are extremely small.

The following examples are merely illustrative of the present invention without being deemed limitative in any manner thereof.

In the examples, the notation of the aluminum alloys is based upon nomenclature according to DIN 1,725. The quality of the oxide layers was determined by the socalled Testal value according to DIN 50,949 and by the loss factor d (Anotest apparatus) according to DIN 50,- 920 (design). Further, the products of the after-sealing were tested by means of the Green test according to DIN 50,146. DIN is the abbreviation for Deutsche Industrie- Norm representing a series of standard German published test procedures.

EXAMPLE 1 Aluminum sections (AlMgSi 0.5) degreased with an aqueous alkali metal hydroxide solution and pickled in the usual way, which were anodically oxidized in the direct current-sulfuric acid process (layer thickness 21 1,) were sealed at 100 C. for 60 minutes with a solution of 0.0005 gm. per liter of citric acid in completely de-salted water, which was adjusted with ammonia to pH 5.7 prior EXAMPLE 2 Aluminum sheets (AlSiS) degreased in the usual way, which were anodically oxidized in the direct currentsulfuric acid oxalic acid process (layer thickness 12 were sealed at 98 C. for 40 minutes with a solution of 0.002 gm. per liter of tartaric acid in distilled water,

which was adjusted to pH 5.8 with ammonia prior to scaling. The sheets showed no sealing film. The Testal value had fallen from over 300 to 14, the loss factor d amounted to 0.48 and the Green test also showed a satisfactory sealing (reflection value 93% Practically the same results were obtained when, instead of the tartaric acid, an equivalent amount of monoor disodium, potassium, ammonium or magnesium tartrate was used with corresponding pH adjustment with ammonia or acetic acid.

EXAMPLE 3 Aluminum sections (AlMg3) degreased with an aqueous alkali metal hydroxide solution and pickled in the usual way, which were anodically oxidized by an autocolor process (layer thickness 30 were sealed at 100 C. for 80 minutes in a solution of 0.01 gm. per liter of the monopotassium salt of d-saccharic acid in completely desalted water, which was adjusted with ammonia to a pH value of 6.0 prior to sealing. The sections showed no sealing film. The Testal value had fallen from over 300 to 4.5 and the loss factor a amounted to 0.51.

EXAMPLE 4 Aluminum sections (AlMgSi0.5) degreased with an aqueous alkali metal hydroxide solution and pickled in the usual way, which were anodically oxidized in the direct current sulfuric acid process (layer thickness 20- 2241..) were sealed at 100 C. for minutes at a pH value of 5.6 (adjusted with ammonia prior to scaling) in solutions which contained the chemicals mentioned below in the amounts given therein. The formation or prevention of the sealing film and the varying quality of the sealing, measured by means of the Testal value, the loss factor 0. and the Green test (in percent of the reflection value), are summarized in Table I. Only when compounds according to the invention were used in suitable concentration did no sealing film result and no harmful effect on the subsequent sealing occur.

TABLE I Green I Concentration, Testal Loss test, Substance Sealing film gm. per liter value factor percent Citric acid. 0. 002 7. 5 0. 47 92 Tartaric acid. 0. 005 10. 0 0. 55 88 Saccharic acid 0.005 0. O 0. 40 J0 Gallic acid... 0. 01 11.0 0.52 85 Do ..d0 0. 05 12.0 0. 00 80 Suceinie acid Not 'Iricarballylie acid d 1 Lactic acid... Glyeeric acid. Gluconie acid Salicylic acid.... Dihydroxybenzoic acid .do 0. 05 Dihydroxymaleic acid- Prevented, damage to oxide layer. 0. 01

Do Not prevented Tartan'e acid Prevented, damage to oxide layer. Citric acid ..d0

to the sealing. The sections showed no sealing film. The layer thickness after the sealing was 21 the Testal value had fallen from over 300 to 7.5 and the loss factor d amounted to 0.52. The Green test showed a satisfactory Although the present invention has been disclosed in connection with a few preferred embodiments thereof, variations and modifications may be resorted to by those skilled in the art without departing from the principles sealing (reflection value 96%, measured as scattered light of the new invention. All of these variations and modifi- 5 cations are considered to be within the true spirit and scope of the present invention as disclosed in the foregoing description and defined by the appended claims.

We claim:

1. In the process for treating the surface of aluminum or an aluminum alloy which comprises subjecting said surface to an anodic oxidation and subsequently sealing with hot water or steam, the improvement which consists essentially of sealing said surface by applying an aqueous solution containing from 0.0005 to 0.05 gm. per liter of a hydroxy carboxylic acid selected from the group consisting of (A) citric acid, tartaric acid, gallic acid and saccharic acid, (B) a water soluble salt of said acids of (A), and (C) the mixtures thereof at a temperature ranging from 90 C. to the solution boiling point temperature and at a pH of from 5 to 6, to the anodic oxidized surface.

2. The process of claim 1 in which the sealing is carried out with said solution containing from 0.0005 to 0.005 gm. per liter of citric acid.

3. The process of claim 1 in which said scaling is carried out at a temperature ranging from about 98 C. to 100 C.

4. The process of claim 1 in which said sealing solution is applied from 1.5 to 3.5 minutes per ,u layer thickness of the anodic oxide layer.

5. The process of claim 1 in which said salt of (B) is selected from the group consisting essentially of alkali metal salts, ammonium salts, and alkaline earth salts.

6. The process of claim 1 in which said sealing is carried out with said solution containing from 0.002 to 0.05 gm. per liter of said hydroxy carboxylic acid.

References Cited UNITED STATES PATENTS 2,008,733 7/1935 Tosterud 204-35 N HOWARD S. WILLIAMS, Primary Examiner R. L. ANDREWS, Assistant Examiner 

